<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>html5</title>
</head>
<body>
    <input type="button" value="点击下" id="btn1">
</body>
</html>
<script>
    document.getElementById("btn1").onclick = function(){
        alert(111);
    }

    setTimeout(function(){
        document.body.innerHTML += "aaaa";
    },5000);
</script>